Thermal inkjet printers have experienced a great deal of commercial success since their inception in the early 1980's. The fundamental principles of how thermal inkjet printers work is analogous to what happens when a pot of coffee is made. Using the electric drip coffee maker analogy, water is poured into a container (reservoir) that has a heating element at its base. Once the coffee has been placed in the filter, the coffee maker is turned on and power is supplied to a heating element that is now surrounded by water. As the heating element reaches a certain temperature, some of the water surrounding it changes from a liquid to a gas, thus, creating bubbles within the water. As these "super heated" bubbles are formed, heated water surrounding these bubbles is pushed from the reservoir into a tube and finally into the carafe. Referring now to the thermal printhead, ink is located in a reservoir that has a heating element (heater resistor) at its base. When the heater resistor is turned on for a certain amount of time (pulsed by electronic circuitry) corresponding to a certain temperature, the ink surrounding the heater resistor changes from a liquid to gas phase, thus, creating a bubble that pushes surrounding ink through an orifice and finally onto a printing medium. The aforementioned example radically simplifies inkjet technology. For a more detailed treatment of the history and fundamental principles of thermal inkjet technology, refer to the Hewlett-Packard Journal, Vol. 36, No. 5, May 1985.
In the coffee maker analogy, only one heating element is turned on to heat the water, whereas, in a thermal inkjet printhead, up to and exceeding 200 heating elements (heater resistors) may be turned on (fired) simultaneously. The simultaneously fired heater resistors can create great demands on the power supply circuitry. Although there are power supply circuits that may supply a constant power under dynamic loading conditions, the application of these systems in inkjet technology is often not practical. Cost, size and the small dimensions of the wires used to distribute power to the heater resistors often confine the application of such power supply systems. Therefore, it is not uncommon in inkjet technology to employ power supply circuits and conductors that are limited in their ability to robustly supply constant power to all heater resistors on the printhead simultaneously. Consequently, in print modes that require all heater resistors to be on, the power may droop (at the heater resistor) to a level insufficient to consistently create liquid to gas phase transformations of the ink. When this occurs, several undesirable characteristics of the printhead are created including but not limited to: (1) The heater resistors may not turn on at the same time, causing inconsistencies in the placement of ink on the writing medium; (2) the heater resistors may partially turn on, creating droplets of varying size and direction; and (3) the time required for the power supply circuitry to respond in preparation for the next firing instruction increases.